Java多线程之synchronized（五）

上篇介绍了用synchronized修饰static方式来实现“Class 锁”，今天要介绍另一种实现方式，synchronized(class)代码块，写法不一样但是作用是一样的。下面我附上一段代码来看一下synchronized(class)代码块的基本用法，如下：

public static void main(String[] args) {

		Service4 s1 = new Service4();
		Service4 s2 = new Service4();
		ThreadA a = new ThreadA(s1);
		ThreadB b = new ThreadB(s2);
		a.setName("A");
		b.setName("B");
		a.start();
		b.start();
	}

	public static class ThreadA extends Thread {

		private Service4 service;

		public ThreadA(Service4 service) {
			super();
			this.service = service;
		}

		@Override
		public void run() {

			super.run();
			service.printA();
		}
	}

	public static class ThreadB extends Thread {

		private Service4 service;

		public ThreadB(Service4 service) {
			super();
			this.service = service;
		}

		@Override
		public void run() {

			super.run();
			service.printB();
		}
	}

}

class Service4 {

	static public void printA() {
		synchronized (Service4.class) {
			try {
				System.out.println("线程：" + Thread.currentThread().getName()
						+ "在" + System.currentTimeMillis() + "进入printA");
				Thread.sleep(3000);
				System.out.println("线程：" + Thread.currentThread().getName()
						+ "在" + System.currentTimeMillis() + "离开printA");
			} catch (InterruptedException e) {

				e.printStackTrace();
			}
		}

	}

	static public void printB() {
		synchronized (Service4.class) {
			System.out.println("线程：" + Thread.currentThread().getName() + "在"
					+ System.currentTimeMillis() + "进入printB");
			try {
				Thread.sleep(3000);
			} catch (InterruptedException e) {

				e.printStackTrace();
			}
			System.out.println("线程：" + Thread.currentThread().getName() + "在"
					+ System.currentTimeMillis() + "离开printB");
		}
	}

}

　　    运行结果如下：synchronized(class)代码块的作用和synchronized static的作用是一样的

       以前我说过，synchronized还可以传入其他的实例对象或者方法的形参，那么我现在要说一种把synchronized(class)和String一起使用的特殊情况，还是用代码讲解，下面我附上一段代码，如下：  

	public static void main(String[] args) {

		Service5 service = new Service5();
		ThreadA a = new ThreadA(service);
		a.setName("A");
		a.start();
		ThreadB b = new ThreadB(service);
		b.setName("B");
		b.start();
	}

	public static class ThreadB extends Thread {
		private Service5 service;

		public ThreadB(Service5 service) {
			super();
			this.service = service;
		}

		@Override
		public void run() {
			// TODO Auto-generated method stub
			super.run();
			service.print("AA");
		}
	}

	public static class ThreadA extends Thread {
		private Service5 service;

		public ThreadA(Service5 service) {
			super();
			this.service = service;
		}

		@Override
		public void run() {
			// TODO Auto-generated method stub
			super.run();
			service.print("AA");
		}
	}
}

class Service5 {
	public static void print(String string) {

		try {
			synchronized (string) {
				while (true) {

					System.out.println(Thread.currentThread().getName());
					Thread.sleep(1000);
				}
			}

		} catch (InterruptedException e) {

			e.printStackTrace();
		}

	}
}

   运行结果如下：可以看到输出结果会打印出无数个连续的A，这是由于在JVM中有String常量池缓存的功能，所以说printA()和printB()两个方法里传进来的“AA”是同一个值，因此两个线程持有相同的锁，所以总有一个线程执行不到。

上面已经看到了常量池带来的问题，因此大多情况下，都不用String最为对像锁，而改用其他的，比如new Object()实例化一个Object对象。可以看看下面的例子，运行后的区别在哪，如下：

	public static void main(String[] args) {

		Service6 service = new Service6();
		ThreadA a = new ThreadA(service);
		a.setName("A");
		a.start();
		ThreadB b = new ThreadB(service);
		b.setName("B");
		b.start();
	}

	public static class ThreadB extends Thread {
		private Service6 service;

		public ThreadB(Service6 service) {
			super();
			this.service = service;
		}

		@Override
		public void run() {
			// TODO Auto-generated method stub
			super.run();
			service.print(new Object());
		}
	}

	public static class ThreadA extends Thread {
		private Service6 service;

		public ThreadA(Service6 service) {
			super();
			this.service = service;
		}

		@Override
		public void run() {
			// TODO Auto-generated method stub
			super.run();
			service.print(new Object());
		}
	}
}

class Service6 {

	public static void print(Object object) {

		try {
			synchronized (object) {
				while (true) {

					System.out.println(Thread.currentThread().getName());
					Thread.sleep(1000);
				}
			}

		} catch (InterruptedException e) {

			e.printStackTrace();
		}

	}
}

　　    运行结果如下：可以看到运行结果是交叉的异步的，说明两个线程持有的锁不是同一把锁。